Transport in Plants

Transport in Plants

1. Xylem

   • Transports water and mineral ions up a plant's stem to leaves
   • Root - xylem found in centre as a x
     • Provides support for root
   • Stem - xylem found toward the outside
     • Reduces bending
   • Leaf - xylem makes up network of veins
     • Supports leaves
   • Adaptations
     • Xylem vessels are long tube like structures
       • Formed from vessel elements joined end to end
       • No end walls
         • Allows water to pass up through middle of cell easily
     • Dead cells, containing no cytoplasm
     • Woody substance lignin thickens cell walls
       • Stops walls collapsing inwards
       • Lignin in spiral patterns
         • Flexibility
         • Prevents stem from breaking
         • Amount of lignin increases as cell gets older
         • Supports xylem walls
     • Small pits in walls where there is no lignin
       • Allows water and mineral ions to move in/out of vessels

2. Phloem

   • Transports dissolved substances - sugars (like sucrose) up and down a plant
   • Root - phloem found in centre outside of xylem
     • Supports root
   • Stem - phloem on outside
     • Reduces bending
   • Leaf - phloem on inside making up network of veins
     • Supports leaves
   • Adaptations
     • Formed of cells arranged in tubes
     • Purely transport tissue
       • Contains phloem fibres, phloem parenchyma, sieve tube elements and companion cells
       • Not used for support
     • Sieve tube elements
       • Living cells forming tube for transporting sugars through plant
         • Joined end to end to form sieve tubes
       • Sieve parts are end walls
         • With holes to allow sugars to pass through them
         • Sieve plates
       • No nucleus, thin layer of cytoplasm and few organelles
         • Sieve tube elements cannot survive on their own
       • Cytoplasm of adjacent cells is connected through holes in sieve plates
     • Companion cells
       • Companion cell for every sieve tube element
         • Helps it survive
         • Carries out living functions for both itself and sieve tube elements
         • Provides energy to actively transport sugars

3. Multicellular organisms

   • Small surface are to volume ratio
   • Uptake water, minerals and sugars
   • Get rid of waste substances
   • Diffusion would be too slow without a transport system

4. Water transport

   • Entering plant
     • 1) Water enters through root hair cells
     • 2) Passes through root cortex and endodermis
     • 3) Reaches xylem
     • Water moves down a water potential gradient
     • Soil around roots has high water potential
     • Leaves have lower water potential
       • Water constantly evaporates from them
     • Water potential difference creates a gradient
       • Keeps water moving through plant from roots to leaves
   • Through roots
     • Water travels by osmosis through roots via root cortex, into xylem by pathways
     • Symplast pathway
       • Goes through living parts (cytoplasm) of cells
       • Cytoplasm of neighbouring cells connect through plasmodesmata
         • Small channels in cell walls
     • Apoplast pathway
       • Goes through non-living parts of cells (cell walls)
         • Walls absorbant = easy diffusion
       • When reaching endodermis cells in root, path blocked by casparian strip
         • Waxy, waterproof strip in cell walls
         • Now takes symplast pathway
         • After this water moves into xylem
       • Useful as water has to go through cell membrane first
         • Control if substances in water get through
   • Through leaves
     • Water leaves xylem and moves into cells by apoplast pathway
     • Water evaporates from cell walls into spaces between cells in leaf
     • Stomata open = water moves out of leaf as water vapour into air
       • Down water potential gradient
       • Transpiration
   • Up plant
     • Water movement from roots to leaves = transpiration stream
     • Cohesion
       • Water molecules stick together
       • Causes column of water in xylem to move upwards as one
     • Tension
       • As water evaporates from leaves, this creates suction at top of xylem
       • Pulls more water up into leaf
     • Movement of water causes more water to enter stem through root cortex cells
     • Adhesion
       • Water molecules attracted to walls of xylem vessels
       • Helps water rise up through vessels
     • Air bubbles in xylem can block column of water from reaching cells

5. Transpiration

   • Evaporation of water from a plant's surface
     • Happens as a result of gas exchange
   • Plant need to open stomata to let in CO2 for photosynthesis, to produce glucose
     • Opening stomata lets water out
   • Higher concentration of water inside leaf than in surrounding air
     • Water moves out down water potential gradient
   • Factors affecting transpiration rate
     • Light
       • More light = faster transpiration rate
       • Stomata open wider the lighter it gets
         • Allowing more water to evaporate
       • Dark = stomata closed
         • Little transpiration
     • Temperature
       • Higher temp = faster transpiration rate
       • Warmer water molecules have more energy = evaporate from cells inside leaf faster
         • Increases water potential gradient between inside and outside of leaf
           • Water evaporates out of leaf faster
     • Humidity
       • Lower humidity = faster transpiration rate
       • Water potential gradient increases between leaf and air if air surrounding plant is dry
         • Lower in air, higher in leaf
         • Moves down water potential gradient
       • Increase in water potential gradient = increases transpiration
     • Wind
       • Increase in wind = faster transpiration rate
       • Air movement blows away water molecules from around stomata
         • Water potential in air is now more lower than leaf
       • Increases water potential gradient
         • Increases rate of transpiration
   • Potometers
     • Estimates transpiration rates
     • Measures water uptake by a plant
     • Use to estimate how different factors affect transpiration rates
     • Process
       • 1) Cut shoot underwater at a slant
         • Prevents air entering xylem which can block water column
         • Increases surface area for water uptake
       • 2) Assemble potometer in water and insert shoot underwater
       • 3) Remove potometer from water, but keeping end of capillary tube in beaker of water
       • 4) Check potometer is watertight and airtight
       • 5) Dry leaves
         • Make sure water potential around leaves isn't higher than it should be
       • 6) Allow time for shoot to acclimatise, then shut tap
       • 7) Remove end of capillary tube from beaker until one air bubble has formed
         • Then put end of tube back into water
       • 8) Record starting position of air bubble
       • 9) Start stopwatch and record distance the bubble moves per unit of time
       • Keep conditions constant throughout experiment
         • Factors etc
   • Xerophytic plants
     • Examples
       • Cacti
       • Pine trees
       • Prickly pears
     • Stomata are sunk in pits
       • Trapping water vapour
         • Reduce transpiration by lowering water potential gradient
     • Hair layer on epidermis
       • Traps moist air round stomata
         • Reduces water potential gradient between leaf and air
     • Curled leaves
       • Traps moist air
       • Lowers exposed surface area for losing water
       • Protects stomata from wind
     • Less stomata
       • Fewer places for water vapour to diffuse out of leaf
     • Thick waxy layer on epidermis
       • Layer is waterproof
         • Water cannot pass through to evaporate
     • Spines for leaves
       • Reduces surface area for water loss

6. Translocation

   • Movement of assimilates in plant
     • Sugars
   • Requires energy
   • Happens in phloem
   • Moves assimilates from source to sink
     • Source = where assimilates are produced
       • High concentration
     • Sink = where assimilates are used up
       • Low concentration
     • Enzymes maintain concentration gradient from source-sink
       • Changing dissolves substances at sink into something else/by breaking them down
         • = lower concentration at sink than source
   • Examples
     • Sucrose
       • Source = leaves
       • Sink = food storage organs and meristmems in roots/stems/leaves
     • Potatoes
       • Sucrose converted to starch in sink
         • Lower concentration of sucrose at sink than in phloem
   • Mass flow
     • Hypothesis
       • 1) Source
         • Active transport loads assimilates into sieve tubes of phloem
         • Lowering water potential inside sieve tubes
           • Water enters tubes by osmosis
             • Creates high pressure inside sieve tubes at source end of phloem
       • 2) Sink
         • Assimilates removed from phloem to be used up
           • Increases water potential inside sieve tubes
             • Lowers pressure inside sieve tubes
       • 3) Flow
         • Result of pressure gradient from source end to sink end
         • Gradient pushes assimilates along sieve tubes to where they are needed
     • Evidence
       • Support
         • Removing ring of bark from woody stem = bulge forms above ring
           • Includes phloem but not xylem
           • Fluid in bulge has higher concentration of sugars than below the ring
             • Sugars cannot move past the area where bark has been removed
               • Evidence downward flow of sugars
         • Investigating pressures in phloem using aphids
           • Aphids pierce phloem, bodies removed, mouthparts left behind allowing sap to flow out
             • Sap flows out quicker nearer leaves than further down stem
               • Evidence for pressure gradient
         • Metabolic inhibitors in phloem = translocation stops
           • Inhibitors stop ATP production
             • Evidence active transport is involved
       • Against
         • Sugar travels to many different sinks
           • Not just to one with highest water potential
             • As model suggests
         • Sieve plates would create barrier to mass flow
           • A lot of pressure would be needed for assimilates to get through at reasonable rate
       • Experiment
         • Showing mass flow hypothesis
         • Two containers A and B
           • Lined with selectively permeable membrane like cells
         • Top tube connecting A and B represents phloem
         • Bottom tube connecting A and B represents xylem
         • A = source end
           • Contains concentrated sugar solution
         • B = sink end
           • Contains weak sugar solution
         • Water enters A by osmosis
           • Increases pressure = sugar solution flows along top tube
         • Pressure increases in B
           • Forcing water out and back through bottom tube